In an image-forming device such as a laser printer, a charger, a developer, and a transfer roller are disposed around a photosensitive drum. When the photosensitive drum is rotated, the surface of the photosensitive drum is uniformly charged by the charger and then selectively exposed by a laser beam. As a result, the electric charge on the surface of the photosensitive drum is partially removed and a latent image is formed on the surface of the photosensitive drum. When the latent image formed on the surface of the photosensitive drum rotates opposite the developer, toner carried on the developer is supplied to the latent image and the latent image is developed into a toner image. Subsequently, when the toner image is rotated opposite a transfer roller, the toner image carried on the photosensitive drum is transferred onto a sheet of paper conveyed between the photosensitive drum and the transfer roller.
In order to form an excellent latent image and toner image on the surface of the photosensitive drum, as well as to transfer the toner image onto a paper with high accuracy, the relative positional relationship between the photosensitive drum and components disposed around the photosensitive drum, such as the charger, is very important in such image-forming device.
An image-forming device disclosed in Japanese Utility Model Publication No. HEI-7-19752 provides a first casing including a photosensitive drum and a second casing including a charger. In the image-forming device, a relative position between the photosensitive drum and the charger is positioned by attaching the first casing and the second casing. The second casing has an engagement portion to be engaged with a shaft of the photosensitive drum. The first casing has a claw portion to be locked a locking hole formed in the first casing. When the engagement portion is engaged with the shaft of the photosensitive drum, and the claw portion is locked the locking hole, engagement between the first and second casings is achieved.